Propulsion System for Highly Maneuverable Airship

ABSTRACT

One embodiment of a propulsion system for omnidirectional maneuverability and efficient forward flight of an airship comprising only fixed, unidirectional engines (17, 19, 20). The thrust vectors of the fixed engines (19, 20) are oriented in a way that their speeds can be chosen such that all forces acting on the airship (i.e., engine thrusts, gravity, buoyancy, wind and potentially others) are together resulting in the desired motion. In one embodiment these are four ducted fans (17) at the bow and four ducted fans (19) at the stern of the aircraft. Their thrust vectors can be decomposed into three vectors of equal length that are each parallel to one of the three axis of a cartesian coordinate system like defined in FIG. 8. In one embodiment efficient forward flight is achieved by an additional engine (20) at the stern of the airship.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority from provisional patent application with Application No. 62/574,532, EFS ID 30707227, Confirmation Number 5395, filed 19 Oct. 2017 by the present inventor, which is incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to ligther-than-air vehicles, and more particularly to their propulsion system.

BACKGROUND Prior Art

The following is a tabulation of some prior art that presently appears relevant:

U.S. Patents

Publication Pat. No. Kind Code Date Patentee U.S. Pat. No. 5,449,129A B64B 1/26 1995 Sep. 12 Lockheed Corp. U.S. Pat. No. 5,026,003 B64B 1/34 1991 Jun. 25 Smith U.S. Pat. No. 4,402,475 B64B 1/26 1983 Sep. 6 Airships Intl., Inc. US 020150078620A1 B64B 1/30 2015 Mar. 19 Disney Res Zurich U.S. Pat. No. 1,457,024 B64B 1/26 1923 May 23 Franzen

Nonpatent Sources

-   Johannes EiBing; discussion in Facebook group “Small Airship Union”;     April 2015;     https://www.facebook.com/groups/smallairshipsunion/436456063183024/ -   Guy Norris, “Hybrid Hopes: An Inside Look At The Airlander 10     Airship”, Aviation Week, May 15, 2015,     http://aviationweek.com/technology/hybrid-hopes-inside-look-airlander-10-airship

The above propulsion systems and most other existing propulsion systems for airships can be divided into three categories:

-   -   Swiveled engines     -   Pitch-adjusted propeller blades     -   Fixed engines         -   in direct air flow         -   in combination with thrust redirecting ducts or tubes         -   in combination with thrust redirecting vanes

Swiveled engines liked described by Lockheed and Disney have two main disadvantages:

-   -   Swiveling and thus changing the direction of thrust takes some         time. Immediately reacting to gusts of wind or other fast         maneuvers are hardly possible that way.     -   Additional actuators are necessary to cause the swivel movement.         Weight, energy consumption and complexity increase compared to         fixed engine concepts.

Pitch-adjusted propeller blades are used to reverse the direction of thrust of a propeller. The mechanical structures required to change the pitch of each blade of a propeller are complex and introduce many additional parts that increase the chance of a system failure. In order to actuate the pitch adjusting gears and levers a separate motor is required which increases weight and energy consumption. Finally the shape of the blades cannot be optimized for producing thrust in the forward direction. Instead a compromise has to be found in order to produce a significant amount of thrust in the reversed direction as well.

The fixed engine concepts known to me, like the one described by Smith, have the disadvantage that they do not allow for a movement of the airship in all six degrees of freedom. Most of these concepts lack the ability to move the airship in the lateral direction (left/right).

The patent by Airships Intl. does describe thrusters at the bow and stern of an airship. However, their use is only intended as control thruster for correcting the attitude of the airship instead of a missing empennage. Using the bow and stern thruster in order to cause a lateral (sidewards crabbing) movement is not considered. Vertical thrusters are described for augmentation of static lift of the airship, but not for increasing maneuverability. Thrusters to propel the airship forwards or backwards are not described. Like that an omnidirectional maneuverability is not achievable by the suggested concept.

The patent by Franzen describes the use of fixed engines mounted inside of ducts leading throughout the entire airship body in order to redirect the thrust of the engines. Franzen does not describe how an omnidirectional maneuverability of the airship is achievable by an arrangement of these ducts. The long ducts cause a lot of friction for the air that is propelled through them, making the propulsion very inefficient. Actuators for opening and closing the ducts are required in order to achieve a resulting thrust in the desired direction increasing complexity, weight and power consumption.

The Airlander, a hybrid airship by Hybrid Air Vehicles Limited, uses fixed engines in combination with thrust redirecting vanes, like shown in an article by Guy Norris. However the engine arrangement used does not allow for a lateral movement of the airship. Tilting the vanes still takes some time, the tilting angle is limited to less than 90° and in the tilted position the vanes are not able to redirect the complete amount of propelled air into the desired direction, thus decreasing the efficiency of the concept.

Johannes EiBing described in loose terms how a propulsion concept with fixed engines could look like that would enable a movement in almost all direction. However this concept again has three significant drawbacks:

-   -   It does not allow for a backwards movement.     -   It only describes the engine orientation to be 45° relative to         the longitudinal and vertical axis of the airship, which is not         ideal for some applications and is not the only orientation         possible.     -   The efficiency of a forward flight is reduced because the thrust         vectors that have to be activated for this motion partially         cancel each other out.

SUMMARY

In order to eliminate the disadvantages of the existing propulsion concepts like those described above, embodiments of a fixed engine concept is described here that enable near-instantaneous movements of an airship in all directions without having to rotate into a special direction first and without having to reverse the thrust of any engine. This is especially new for movements in lateral directions, i.e., for movements in the yz-plane of the cartesian coordinate system relative to the airship (see, e.g., illustration 1, FIG. 7). The advantages of this near-instantaneous omnidirectional maneuverability is the ability to closely follow a pre-defined path in any direction and to precisely hold a position even under windy conditions. This is an ability that has previously only been achieved by multirotor aerodynes.

In further embodiments, high maneuverability and efficient forward flight are both possible, as opposed to previous engine concepts that involved a trade-off between these two objectives. These flight characteristics are achieved for example in a first embodiment of the propulsion system that uses eight ducted fans 17 and 19 and a stern engine 20 (see illustration 1, FIG. 7).

All embodiments have in common that the thrust vector of each engine is pointing in one fixed direction, and at all times engine speeds can be chosen such that all forces acting on the airship (i.e., engine thrusts, gravity, buoyancy, wind and potentially others) are together resulting in the desired motion.

BRIEF DESCRIPTION OF THE DRAWINGS

I have included 6 illustrations:

Illustration 1 includes FIG. 7 which shows the engine concept on an airship in a three dimensional perspective.

Illustration 2 includes FIG. 8 which shows the thrust vector F of one of the eight ducted fans in its three dimensional orientation and its projections in x, y and z direction that each have magnitude |F/√3|.

Illustration 3 includes FIGS. 9A, 9B, 10A and 10B which show the following orthographic projections of the engine arrangement on the airship: back, front, side and top.

Illustration 4 includes FIGS. 11A, 11B, 12A and 12B which show the active engines in the top view for movements to the left, right, back and front respectively.

Illustration 5 includes FIGS. 13A and 13B, which show the active engines in the side view for the upwards and downwards movement respectively. Further included are the FIGS. 14A and 14B, which show the active engines in the top view for the yaw movement to the left and to the right respectively.

Illustration 6 includes FIGS. 15A and 15B, which show the active engines in the side view for the movements pitch down and pitch up respectively. Further included are the FIGS. 16A and 16B, which show the active engines in the front view for the roll movements to the left and to the right respectively.

LIST OF REFERENCE NUMERALS

-   17 ducted fans (in streamlined housings at bow of airship) -   17BUR ducted fan at the bow upper right position -   17BUL ducted fan at the bow upper left position -   17BLR ducted fan at the bow lower right position -   17BLL ducted fan at the bow lower left position -   18 duct for inlet and outlet of air in streamlined engine housing -   18BLL duct for inlet and outlet of air at the bow lower left     position -   19 ducted fans (in streamlined housings at stern of airship) -   19SUR ducted fan at the stern upper right position -   19SUL ducted fan at the stern upper left position -   19SLL ducted fan at the stern lower left position -   19SLR ducted fan at the stern lower right position -   20 stern engine -   21 fins -   22 gondola -   23 sensor -   24 hull

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Static Structure and Function

The figures and illustrations are given with a first embodiment and a simple version of both model and control system for the propulsion system in mind. The latter that is described in the sub-subsection “Simplified Motion Control” of subsection “Hover Mode” in section “Operation”.

Illustration 1

In FIG. 7 it is shown that there are nine engines mounted to the hull 24. These are eight ducted fans 17 and 19 and one propeller engine 20 at the stern. There are two groups of ducted fans, which are four ducted fans 17 near the bow and four ducted fans 19 near the stern of the airship hull 24. One of the four ducted fans at the stern is not numbered because it is not visible in the three dimensional view of FIG. 7.

The ducted fans 17 and 19 each consist of a housing in the shape of a half-drop that contains the duct for the inlet and outlet of air (see, e.g., 18BLL) as well as rotor blades and a motor that are both mounted inside the duct.

Each of the ducted fans has a unique three-letter name that describes their locations on the hull 24 as seen from standing behind the stern of the airship. The ducted fans 17 near the bow have the following names:

BUR: Bow Upper Right

BUL: Bow Upper Left

BLR: Bow Lower Right

BLL: Bow Lower Left

The ducted fans 19 near the stern have the following names:

SUR: Stern Upper Right

SUL: Stern Upper Left

SLR: Stern Lower Right (see Illustration 3, FIG. 9A)

SLL: Stern Lower Left

There are four identical fins 21 with rudders (not shown) which are mounted on the streamlined housings of the ducted fans 19 near the stern. The advantage of mounting the fins 21 directly onto the streamlined housings is, that the housings are serving the additional purpose of supplying a rigid mounting surface with a larger surface area resting on the hull 24 than the cross-section of the fins 21 would supply. This way tensioning ropes between the fins and the hull, like they are common in the prior art, can be omitted and the drag forces that these ropes would cause in flight do not occur.

Furthermore there is a gondola 22 attached to the bottom of the hull 24 and a sensor 23 attached to the gondola 22.

Illustration 2

In FIG. 8 the thrust vector F of the ducted fan 17BUR is shown as an arrow pointing from the outlet of the ducted fan housing in the direction into which the air is blown by that ducted fan. Each thrust vector points in one of the eight possible directions from {−1,1}³. In other words, the thrust vector F can be decomposed into three vectors of length |F/√3| (F divided by the square root of 3) that are each parallel to one of the three axis of a cartesian coordinate system like defined in the lower left corner of FIG. 8. As the arrangement of ducted fans 17 and 19 is symmetrical, each ducted fan is positioned and oriented symmetrically to 17BUR. This again means that the thrust vector F of each ducted fan 17 and 19 can be decomposed into three components that act in the three directions of a cartesian coordinate system and that have the same magnitude |F/√3|.

Illustration 4

FIG. 11A shows two of the four thrust vectors that are necessary to move the airship to the left, i.e., in the direction −y of the cartesian coordinate system. The ducted fans activated for this movement are 17BUL, 17BLL, 19SUL and 19SLL as seen in FIGS. 7 and 9A.

FIG. 11B shows two of the four thrust vectors that are necessary to move the airship to the right, i.e., in the direction y of the cartesian coordinate system. The ducted fans activated for this movement are 17BUR, 17BLR, 19SUR and 19SLR as seen in FIGS. 7 and 9A.

FIG. 12A shows two of the four thrust vectors that are necessary to move the airship backwards, i.e., in the direction x of the cartesian coordinate system. The ducted fans activated for this movement are 19SUL, 19SUR, 19SLL and 19SLR as seen in FIGS. 7 and 9A.

FIG. 12B shows two of the four thrust vectors that are necessary to move the airship forwards, i.e., in the direction −x of the cartesian coordinate system. The ducted fans activated for this movement are 17BUL, 17BUR, 17BLL and 17BLR as seen in FIGS. 7 and 9A.

Illustration 5

FIG. 13A shows two of the four thrust vectors that are necessary to move the airship upwards, i.e., in the direction z of the cartesian coordinate system. The ducted fans activated for this movement are 17BLL, 17BLR, 19SLL and 19SLR as seen in FIGS. 7 and 9A.

FIG. 13B shows two of the four thrust vectors that are necessary to move the airship downwards, i.e., in the direction −z of the cartesian coordinate system. The ducted fans activated for this movement are 17BUL, 17BUR, 19SUL and 19SUR as seen in FIGS. 7 and 9A.

FIG. 14A shows two of the four thrust vectors that are necessary to yaw the airship to the left, i.e., rotate it around the z-axis of a cartesian coordinate system, whose origin is located near the center of the airship hull 24, in the direction of ψ (psi). The ducted fans activated for this movement are 17BUL, 17BLL, 19SUR and 19SLR as seen in FIGS. 7 and 9A.

FIG. 14B shows two of the four thrust vectors that are necessary to yaw the airship to the right, i.e., rotate it around the z-axis of a cartesian coordinate system, whose origin is located near the center of the airship hull 24, in the direction of −ψ (minus psi). The ducted fans activated for this movement are 17BUR, 17BLR, 19SUL and 19SLL as seen in FIGS. 7 and 9A.

Illustration 6

FIG. 15A shows two of the four thrust vectors that are necessary to pitch the airship down, i.e., rotate it around the axis that is parallel to the y-axis and goes approximately through the center of gravity of the airship, in the direction of θ (theta). The ducted fans activated for this movement are 17BLL, 17BLR, 19SUL and 19SUR as seen in FIGS. 7 and 9A.

FIG. 15B shows two of the four thrust vectors that are necessary to pitch the airship up, i.e., rotate it around the axis that is parallel to the y-axis and goes approximately through the center of gravity of the airship, in the direction of −θ (minus theta). The ducted fans activated for this movement are 17BUL, 17BUR, 19SLL and 19SLR as seen in FIGS. 7 and 9A.

FIG. 16A shows two of the four thrust vectors that are necessary to roll the airship left, i.e., rotate it around the axis that is parallel to the x-axis and goes approximately through the center of gravity of the airship, in the direction of −ϕ (minus phi). The ducted fans activated for this movement are 17BUL, 17BLR, 19SUL and 19SLR as seen in FIGS. 7 and 9A.

FIG. 16B shows two of the four thrust vectors that are necessary to roll the airship right, i.e., rotate it around the axis that is parallel to the x-axis and goes approximately through the center of gravity of the airship, in the direction of ϕ(phi). The ducted fans activated for this movement are 17BUR, 17BLL, 19SUR and 19SLL as seen in FIGS. 7 and 9A.

Operation

A first embodiment of the propulsion system enables two flight modes: a cruise mode and a hover mode, which are explained in detail below.

Cruise Mode

In the cruise mode the airship is propelled in the forward direction by the stern engine 20. Maneuvering is accomplished by deflecting the rudders of the fins 21. Compared to using the ducted fans 19, this enables a very energy efficient flight for the following reasons:

-   -   The stern propeller rotates around the longitudinal axis         (parallel to the x-axis) so that its thrust vector is completely         used for forward propulsion.     -   The propulsion efficiency of the stern propeller is increased by         wake effects at the stern.     -   The stern propeller can have a rather large diameter without         having to mount it on a structure that would have to prevent it         from interfering with the hull. A large diameter for the stern         propeller is advantageous because it can rotate slower than a         smaller-diameter propeller while producing the same amount of         thrust. Motors with a lower rpm/V (revolutions per minute per         Volt) can be used at a higher voltage, which leads to a lower         current draw. The lower current draw leads to a smaller voltage         drop and to smaller heat losses across the motor wires, which         ultimately enables a more efficient operation.     -   Maneuvering by using the rudders is more energy efficient         compared to using propeller thrust, because comparably small         servo motors are sufficient to actuate the rudders. These only         have to be actuated from time to time to correct the flight path         and do only consume relatively little energy.

Hover Mode

In the hover mode the airship is propelled by the eight ducted fans 17 and 19 and optionally also the stern engine 20. At every time in this mode, all engine speeds are chosen such that all forces acting on the airship (i.e., engine thrusts, gravity, buoyancy, wind and potentially others) are together resulting in the desired motion. A video of a working prototype for the hover mode can be found on the Youtube channel of aerobotX Inc. (https://www.youtube.com/channel/UCBQ6dG1xubKgnXz9xdFHL1A) e.g. under the title “ObliX mini with Computer Vision”: https://youtu.be/pvzrx_ZGC54

Advanced Motion Control

In broadest terms, the propulsion system relies on two techniques to achieve the desired motion: First, a (mathematical) model that describes the relationship between engine thrusts and the resulting motion. Second, a closed-loop (feedback) control system that compensates for model inaccuracies relative to the real world.

For illustration, two examples for both techniques are described:

-   -   1. First, a rigid airship (but also a blimp with a fully         inflated hull) as a rigid body that behaves according to the         laws of classical mechanics may be modeled. In classical         mechanics, movement of a rigid body has six degrees of freedom:         That is, any motion can be decomposed into its translational         part (consisting of three components, for x/y/z-axis) and its         rotational part (consisting again of three components, for         roll/pitch/yaw about a fixed local axis, typically chosen as         running through the center of gravity). Translational         acceleration is determined by Newton's second law of motion         (“F=m·a”), and rotational acceleration is determined by its         angular analog, plus Euler's equations. Thus, by solving a         system of equations, the necessary engine thrusts can be         computed from the desired acceleration. The system of equations         is guaranteed to have a solution in all directions because there         are 8 engines, but only 6 degrees of freedom.     -   2. Second, a well-known technique for a closed-loop control         system is a PID controller. There could be six PID controllers         on the airship, one each for controlling longitudinal velocity,         lateral velocity, vertical velocity, yaw velocity, roll, and         pitch.

Such an implementation may be chosen if computer-supported flight control and avionics are desired, and if there is sufficient compute power to continuously solve systems of equations in real time.

Simplified Motion Control

In an environment where microcontrollers are either not feasible or not desirable, simpler implementations of both model and control system are possible. For instance:

-   -   1. First, a less precise model may be chosen where desirable         symmetries are just assumed to be present. For instance, one         could assume that equal thrusts in the upper stern and lower         stern engines entirely cancels out any torque. While this is         clearly a simplifying assumption, with no chance of precisely         describing the real-world behavior, it may in some use cases be         sufficiently close to the real-world behavior.     -   2. Second, the “control system” may as well be embodied by a         human pilot.

The figures and illustrations are given with this simple implementation of the control system in mind.

Advantages of Ducted Fans

In the first embodiment of the propulsion system ducted fans are used instead of big propellers because their small size does not cause much additional air resistance and their housing guards the fans from interference with objects or people. In addition a vortex ring around the tip of the fan blades is avoided by the operation in the duct, thus making propulsion more efficient. Guarding the fans is more important than guarding the stern propeller because the ducted fans are used when precise maneuvering is necessary in confined spaces, close to objects or above people. In such a situation the stern propeller can be switched off in order to reduce the risk of damage or injury.

Operation of Ducted Fans

The maneuverability in all directions is achieved by the orientation of the ducted fans 17 and 19. The stern engine 20 can be helpful with station keeping or moving against the wind, especially when the bow of the airship is pointing against the direction of the wind.

As shown in the example for one ducted fan in FIG. 8, the thrust vector F produced by the ducted fan is pointing in the direction of the airflow and is oriented in such a way that it has a component in every direction x, y and z of a cartesian coordinate system. In this embodiment all components have the same magnitude |F/√3| (F divided by the square root of 3). The eight thrust vectors F of the eight ducted fans are arranged symmetrically such that a combination of running fans can be found in which two of the three components x, y, z of the thrust vectors F cancel each other out while the third components add up in a resulting thrust vector. Illustrations 4, 5 and 6 show these combinations for every direction x, y and z as well as for every rotation V, (I) and 0 (psi, phi and theta) as defined by the coordinate system in the respective figures. In addition to the figure descriptions above, one translational and one rotational movement will be explained in detail in the following:

As shown on Illustration 4 in FIG. 11A, in order for the airship to move left, i.e., in the direction −y, the ducted fans 17BUL, 17BLL, 19SUL and 19SLL are activated to produce a force of magnitude |F_(res)|. The x-components of 17BUL and 19SUL as well as the x-components of 17BLL and 19SLL act in opposite directions and cancel each other out. The z-components of 17BUL and 17BLL as well as 19SUL and 19SLL also act in opposite directions and cancel each other out. The y-components of 17BUL, 17BLL, 19SUL and 19SLL all act in the same direction and add up to a resulting thrust of magnitude F_(res)=4*|F/√3| (4 times F divided by the square root of 3), which points in the direction parallel to the y-axis. The symmetry of the airship design ensures that this resulting thrust introduces only negligible rotation.

As shown on Illustration 5 in FIG. 14A, in order for the airship to rotate around its vertical axis, i.e. in the direction of ψ (psi) around an axis parallel to the z-axis, the ducted fans 17BUL, 17BLL, 19BUR and 19BLR are activated. Their thrust vectors F can each be written as sum of three orthogonal vectors F_(pll), F_(orth) and F_(z) where:

-   -   F_(pll) is pointing in the direction of the dotted line that         connects 17BUL and 19SUR on the top and 17BLL and 19SLR on the         bottom of the airship respectively (not shown).     -   F_(orth) is pointing in the direction that is orthogonal to         F_(pll) in the xy-plane.     -   F_(z) is pointing in the direction of the z-axis (not shown).

The thrust vectors F_(z) of 17BUL and 17BLL as well as the thrust vectors F_(z) of 19BUR and 19BLR cancel each other out. The thrust vectors F_(pll) of 17BUL and 19SUR as well as the thrust vectors F_(pll) of 17BLL and 19SLR cancel each other out. Only the thrust vectors F_(orth) remain and add up to the resulting momentum of rotation M_(res) (see FIG. 14A) around an axis that is parallel to the z-axis and goes through the center of the airship.

The movements described above are the elementary movements along and about the three axis of the cartesian coordinate system. The translations in all other directions and the rotations about all other axes are achieved by the linear combination of the elementary movements.

For example, if the thrust vectors for the upwards movement (see FIG. 13A) and the thrust vectors for the forward movement (see FIG. 12B) are combined, the resulting movement will be both upward and forward, with the airship staying level.

Symmetry Conditions

The symmetrical arrangement of ducted fans 17 at the bow and 19 at the stern (compared to only having engines at the stern or at the bow) in the embodiment described above serves the purpose of preventing a rotation about the center z-axis of the airship for thrust in the y-direction as well as a rotation about the center y-axis of the airship for thrust in the z-direction.

The symmetrical arrangement of ducted fans relative to top and bottom of the airship (compared to only having engines at the top or at the bottom) in the embodiment described above serves the purpose of preventing a rotation about the center x-axis of the airship for thrust in the y-direction as well as a rotation about the center y-axis of the airship for thrust in the x-direction.

CONCLUSION, RAMIFICATIONS AND SCOPE

Accordingly, the reader will see that the proposed propulsion system enables an airship to move in all six degrees of freedom as well as hold a defined position and orientation. This is possible even in windy conditions. The fixed direction of thrust for every engine allows for a near-instantaneous reaction to the control commands because only the magnitude of thrust has to be adjusted. This is much faster than having to swivel an engine, more energy efficient than having to redirect the direction of thrust by mechanical means like blinds or ducts and mechanically simpler and thus more reliable compared to swiveling an engine, blinds, ducts or the blades of a propeller. One embodiment also enables an efficient forward flight by an additional engine at the stern of the airship and the use of conventional rudders. This way a so called “Hover Mode” like known from helicopters and a so called “Craze Mode” like known from airplanes can both be combined in order to achieve long flight times and high maneuverability with one aircraft.

While my above description contains many specificities, these should not be construed as limitations on the scope, but rather as an exemplification of one embodiment thereof. Many other variations are possible. For example:

-   -   1. The airship can have an arbitrary shape as long as engines         can be arranged such that combined engines thrusts can produce         any required resulting force and torque. To achieve full         6-degree-of-freedom movement this means at least 6 engines with         sufficiently different thrust vectors (due to well-known results         from linear algebra). Also see chapter “Hover Mode”.     -   2. A ducted stern propeller can be used for increased efficiency         and safety.     -   3. The thrust vectors of the ducted fans can point into         different directions, i.e., individually, in groups or all of         them. This way different magnitudes for the x, y and z         components of the thrust vectors will lead to different         resulting forces in the directions of movement.         -   3.1. For example, if the y-component of all thrust vectors             of the ducted fans 17 and 19 is increased in the same way,             then the resulting force for the movement in the y-direction             will be bigger while the resulting force in the x and/or the             z directions will be smaller.         -   3.2. Another example could be to increase the y-components             of the thrust vectors only for the four ducted fans 19 at             the stern. This could improve movement in lateral direction             by taking into account that the lateral drag at the stern is             bigger than at the bow because of the empennage. A more             balanced movement would be the result.             -   The direction of the thrust vector of a ducted fan is                 only limited by the hull. Expelled air should not hit                 the hull or flow in close proximity to it in order to                 avoid surface-induced disturbances of the air flow.         -   3.3. As a last example, the direction of all thrust vectors             can be inverted. This would help avoiding the Coandă-Effect             by leading the expelled air away from the hull better,             because the vectors would point away from it rather than             over it.     -   4. The eight engines 17 and 19 do not have to be ducted fans and         the stern engine does not have to be a propeller. Any other kind         and size of engines can also be used. The importance lies in the         fixed direction of the thrust vectors of all engines.     -   5. The stern engine 20 can be omitted, especially when efficient         forward flight is not required.     -   6. The rudders on the fins 21 can be omitted and turns of the         airship can be initiated by the ducted fans 17 and 19 only. On         the one hand this would require more energy, but on the other         hand it would also reduce weight for the rudders and their         actuation.     -   7. The fins 21 of the airship can be omitted. In this embodiment         the ducted fans 17 and 19 have to replace the function of the         fins 21 and stabilize the airship. On the one hand this would         reduce the drag force induced by the fins 21 and safe weight. On         the other hand it would increase the energy consumption in most         practical flight situations because the passive fins can be more         efficient in stabilizing the airship in the direction of flight         than active propulsion means.     -   8. As a combination of the embodiments (5.) and (7.) the stern         engine 20 and the fins 21 can be omitted. In this embodiment the         ducted fans 17 and 19 have to cause and stabilize the movement         of the airship. This is the most energy demanding mode of all         embodiments proposed.     -   9. For a simplified flight control, like described in the         subchapter “Simplified Motion Control” of chapter “Hover Mode”         the position of the eight ducted fans can be varied as long as         the symmetry conditions described in chapter “Symmetry         Conditions” are maintained. Especially the distance of the         ducted fans relative to the center of buoyancy of the airship         can be decreased along the x-axis.     -   10. The at least one engine that is producing thrust in the         longitudinal (forward) direction of the airship and that is         embodied by the the stern engine 20 in the first embodiment, can         be mounted to different positions on the airship in further         embodiments:         -   10.1. A bow engine could replace stern engine 20.         -   10.2. In addition to the stern engine 20 an engine at the             airship's bow tip can be used in order to increase forward             thrust and flight stability.         -   10.3. At least one engine could be mounted to the gondola 22             of the airship.         -   10.4. At least one engine could be mounted to a position             along the hull 24 of the airship.

Thus the scope of the embodiments should be determined by the appended claims and their legal equivalents, rather than by the examples given. 

I claim:
 1. A propulsion system for an airship comprising at least six engines rigidly attached to said airship and the at least six thrust vectors of said engines fixedly oriented in a way that the magnitude of their thrust can be chosen such that all forces acting on the airship are together resulting in any desired motion in the six degrees of freedom.
 2. Propulsion system of claim 1 wherein the number of said engines is eight.
 3. Propulsion system of claim 2 wherein all of said eight engines are arranged around the radial circumference of the hull of said airship and four of said eight engines are located closer to the bow of the airship and the other four of said eight engines are located closer to the stern of the airship.
 4. Propulsion system of claim 3 wherein the thrust vector of the engine that is located at the upper right quarter of the bow is pointing in the direction with the vector components x=a*1, y=b*−1, z=c*1, the thrust vector of the engine that is located at the upper left quarter of the bow is pointing in the direction with the vector components x=a*1, y=b*1, z=c*1, the thrust vector of the engine that is located at the lower right quarter of the bow is pointing in the direction with the vector components x=a*1, y=b*−1, z=c*−1, the thrust vector of the engine that is located at the lower left quarter of the bow is pointing in the direction with the vector components x=a*1, y=b*1, z=c*−1, the thrust vector of the engine that is located at the upper right quarter of the stern is pointing in the direction with the vector components x=d*−1, y=e*−1, z=f*1, the thrust vector of the engine that is located at the upper left quarter of the stern is pointing in the direction with the vector components x=d*−1, y=e*1, z=f*1, the thrust vector of the engine that is located at the lower right quarter of the stern is pointing in the direction with the vector components x=d*−1, y=e*−1, z=f*−1, the thrust vector of the engine that is located at the lower left quarter of the bow is pointing in the direction with the vector components x=d*−1, y=e*1, z=f*−1, relative to a cartesian coordinate system who's x-axis is pointing parallel to the longitudinal axis of the airship towards the stern of the airship, who's y-axis is pointing parallel to the lateral axis of the airship towards the right side in the direction of forward travel and who's z-axis is pointing parallel to the vertical axis of the airship towards the upper side of the airship, where * is used to express an arithemtic multiplication and a, b, c, d, e, and f are rational numbers that can be positive or negative but not zero.
 5. Propulsion system of claim 4 wherein said a, c, d, and f take the value −1 and said b and e take the value −2.
 6. Propulsion system of claim 1 wherein at least one of said engines are ducted fans mounted inside of a streamlined housing.
 7. Propulsion system of claim 6 wherein said airship possesses four fins and each of said streamlined housings at the stern of said airship serves as mounting and stabilization means for one of said four fins.
 8. Propulsion system of claim 1 wherein there is at least one engine of the said at least six engines who's thrust vector is approximately oriented into the direction of forward flight in order to enable an efficient flight in the forward direction.
 9. Propulsion system of claim 8 wherein said at least one engine who's thrust vector is approximately oriented into the direction of forward flight is an engine at the stern of the airship.
 10. A propulsion system for an airship comprising nine engines rigidly attached to said airship whereof eight engines are ducted fans and the ninth engine has a propeller with a larger diameter than the propellers of said ducted fans.
 11. Propulsion system of claim 10 wherein said ducted fans are attached to the hull of said airship in a way that four of said ducted fans are located closer to the bow of said airship and the other four of said ducted fans are located closer to the stern of said airship and in a way that all of said ducted fans are equally distributed around the radial circumference of said hull, such that in each quarter of the circumference of said hull there is one of said ducted fans that is closer to the bow and one of said ducted fans that is closer to the stern of said airship.
 12. Propulsion system of claim 11 wherein said ninth engine is mounted to the stern of said airship and its thrust vector is approximately pointing in the direction (x, y, z)=(1, 0, 0) relative to the cartesian coordinate system of said airship.
 13. Propulsion system of claim 12 wherein the thrust vector of the ducted fan that is located at the upper right quarter of the bow is pointing in the direction (x, y, z)=(a*1, b*−1, c*1), the thrust vector of the ducted fan that is located at the upper left quarter of the bow is pointing in the direction (x, y, z)=(a*1, b*1, c*1), the thrust vector of the ducted fan that is located at the lower right quarter of the bow is pointing in the direction (x, y, z)=(a*1, b*−1, c*−1), the thrust vector of the ducted fan that is located at the lower left quarter of the bow is pointing in the direction (x, y, z)=(a*1, b*1, c*−1), the thrust vector of the ducted fan that is located at the upper right quarter of the stern is pointing in the direction (x, y, z)=(d*−1, e*−1, f*1), the thrust vector of the ducted fan that is located at the upper left quarter of the stern is pointing in the direction (x, y, z)=(d*−1, e*1, f*1), the thrust vector of the ducted fan that is located at the lower right quarter of the stern is pointing in the direction (x, y, z)=(d*−1, e*−1, f*−1), the thrust vector of the ducted fan that is located at the lower left quarter of the bow is pointing in the direction (x, y, z)=(d*−1, e*1, f*−1), relative to a cartesian coordinate system of said airship, where * is the symbol for a multiplication, a, b, c, d, e, and f are rational numbers that can be positive or negative but not zero and the direction vectors are meant as approximate vectors.
 14. Propulsion system of claim 13 wherein said b and e each take the value −2 and said a, c, d, and f each take the value −1.
 15. Propulsion system of claim 11 wherein said ducted fans are attached to said hull by being mounted inside of streamlined housings and said streamlined housings being shaped in a way that reduces their drag force in the direction of forward flight.
 16. Propulsion system of claim 15 wherein said airship possesses four fins and each of said streamlined housings at the stern of said airship serves as mounting and stabilization means for one of said four fins. 